1. Field of the Invention
The present invention generally relates to automatic location identification (ALI) systems for locating cellular telephonesxe2x80x94and hence cellular telephone customersxe2x80x94while the customer is placing a telephone call to an emergency dispatch center (i.e., a xe2x80x9c911xe2x80x9d call), so that rescue vehicles can rapidly and accurately locate the caller.
2. Background
Cellular telephone systems are well known in the art, and minimally include an array of cell towers, each tower facilitating duplex communication of cellular telephone customers within a defined region called a xe2x80x9ccell.xe2x80x9d The switching and processing (including signaling and control) of calls within a cellular telephone system and between a Central Office (CO) of a Public Switched Telephone Network (PSTN), is handled by a Mobile Traffic Switching Office (MTSO).
When typical land-based telephone customers place emergency calls (i.e., dial xe2x80x9c911xe2x80x9d and the like) to emergency dispatch centers, the Calling Line Identification (CLI) of the telephone line indicates with a high degree of accuracy, the location of the caller. This enables the efficient and accurate dispatching of public safety personnel and vehicles (e.g., ambulance, fire, police, etc.) to assist those in need of emergency services, regardless of whether the caller knows his/her exact location.
The task of automatically locating a cellular telephone customer is made more difficult by the transient nature of calling customers, and the lack of physical telephone lines (that a CLI would identify). Additionally, cellular telephone customers are often unable to clearly identify and describe their physical location during calls to emergency dispatch centers.
Older prior art methods for locating a cellular telephone customer identify the face of a cell tower in communication with the customer by the tower broadcasting to the MTSO, a Pseudo Automatic Number Identification (ANI). A face is principally responsible for communicating in a cone-shaped region within the boundaries of the cell. The angle (in degrees) of the cone can be given by
360÷n
where n is the number of tower faces of the particular cell tower. For example, if there are 6 tower faces, the cellular customer is assumed to be within a 60xc2x0 isosceles triangular region starting at the cell tower and ending at the cell xe2x80x9cboundary.xe2x80x9d When the cell covers a large area, the height of the triangular region defined by a tower face can be so large as to make locating the customer impractical.
Newer, more sophisticated approaches are being developed which include measuring the propagation time of signals, and converting those times to distances from the cell tower face. These approaches, however, are hardware-and software-intensive, and may be rendered ineffective by environmental factors.
There is an important need to provide an ALI system which gives a more specific location of cellular telephone callers than a vast, cone-shaped region, and which is simpler and less expensive than resource-intensive approaches. Alternative to replacing complex, resource-intensive approaches currently under development, there is a need for a simple, reliable backup system.
In view of the aforementioned shortcomings of the prior art, the Automatic Location Identification system of the present invention compares the electronic footprint of an emergency wireless telephone call with field strength data stored for the face of the cell tower in communication with the caller to ascertain the coordinates of a limited size location polygon.
More particularly, the present invention provides, in a wireless telephone network, an Automatic Location Identification (ALI) method at least including the following steps: Via a cell tower, connecting a telephone call from a wireless telephone customer to a wireless service provider, and transmitting from the cell tower to the wireless service provider, a Pseudo Automatic Number Identification (ANI) representing cell tower indicia; and, via the wireless service provider, identifying a Directory Number (DN) for the caller, and comparing the signal footprint, including field strength aspects, of the customer""s call to stored field strength characteristics associated with the Pseudo ANI.
The method also at least includes the steps of, based upon the outcome of the comparing step, ascertaining the coordinates of a location polygon subsuming the location of the wireless telephone customer, connecting the wireless telephone customer to an emergency dispatch center designated for handling calls emanating from a region subsuming the location polygon when the customer has designated the call as an emergency call, and transmitting to the emergency dispatch center, the DN, and the coordinates of the location polygon.
The present invention also provides, in a wireless telephone network, an ALI system at least including a wireless service provider adapted to handle general switching and call processing of wireless telephone calls, and adapted to identify a DN for the caller, and a plurality of cell towers adapted to connect a telephone call from a wireless telephone customer to the wireless service provider, the cell towers being adapted to transmit a Pseudo ANI representing cell tower indicia.
The wireless service provider further includes a control unit, a comparator, a transmitter, a receiver, and a field strength database storing field strength information related to a plurality of Pseudo ANIs. The comparator is adapted to, under the control of the control unit, compare the signal footprint, including field strength aspects, of the customer""s call to stored field strength characteristics associated with the Pseudo ANI. The control unit is adapted to, based upon the output of the comparator, ascertain the coordinates of a location polygon subsuming the location of the wireless telephone customer.
The wireless service provider is adapted to connect the wireless telephone customer to an emergency dispatch center designated for handling calls emanating from a region subsuming the location polygon. The ALI system transmits to the emergency dispatch center, the DN, and the coordinates of the location polygon.